Project Summary/Abstract: Because of disappointing outcomes of recent clinical trials of anti-amyloid therapy in Alzheimer?s Disease (AD), the field is increasingly interested in elucidating alternative disease mechanisms which may lead to new therapeutic targets that are complementary to anti-amyloid treatment. Extensive literature using post-mortem tissue has suggested that damage to the blood-brain-barrier (BBB) is intricately involved in the pathogenesis of AD. Furthermore, recent studies in animal models suggested a direct link between BBB damage and accumulation of amyloid plaques, in that aggregation of the amyloid protein may be part of an inflammatory response of the brain to pathogen entry, presumably following BBB leakage. However, in vivo studies of BBB in AD are scarce. A method commonly used to evaluate BBB permeability in humans is by administering Gadolinium (Gd) based contrast agent while measuring and modeling dynamic contrast-enhanced (DCE) MRI signal. However, contrast-enhanced MRI is not a common procedure in AD research and has not been used in large-scale or multi-site studies. Therefore, a non-contrast technique to assess BBB permeability is of particular importance in AD research and, if successful, can feasibly translate to clinical screening and monitoring of treatment. The central goal of this application is to develop an MRI technique to measure BBB permeability to water, without using any exogenous agent. MRI can probe water BBB permeability by determining what fraction of the incoming arterial water enters the brain and what fraction remains in the vessel and drains to the vein. This project consists of three logical aims. Aim 1 will develop novel MRI pulse sequences to quantitatively evaluate permeability-surface-area product (PS) of BBB in both global and regional fashion. Aim 2 will validate the non- contrast method with Gd-contrast based technique in humans and with fluorescent microscopy in animal models following osmotic opening of BBB. Aim 3 will conduct clinical application of the technique in elderly individuals who have an established genetic risk to develop Alzheimer?s Disease (AD), i.e. APOE4-carriers. We will compare BBB permeability with amyloid burden and cognitive function, and study their causal relationship through a mediational model analysis. We will also compare the non-contrast permeability results to those using an invasive method of CSF sampling as well as using DCE MRI. The impact of this work is that we will develop a novel non-contrast technique to evaluate BBB permeability in humans. The technique can be completed within 5 minutes on a standard 3T MRI. The outcome of the measurement is in physiological unit of ml water/100g brain/min, thus can be feasibly compared across sites or modalities. This technique will have broad clinical utility, as injury of BBB is implicated in many brain diseases. In this application, we will demonstrate the utility of this technique in AD.